Optical technology is progressing rapidly. Growing needs, particularly in the telecommunications industry, are driving this progress and there is currently a major impetus to improve existing optical systems and to develop new ones. Unfortunately, several major components still are not completely meeting manufacturing yield, field reliability, and operating capacity requirements. These failings have resulted in high costs in existing systems and are limiting the adoption of future systems. One such component is the optical grating.
FIG. 1a-b (background art) depict two variations of traditional gratings. As can be seen, the shape of the groove can vary. FIG. 1a shows square steps and FIG. 1b shows blazed triangles, but other shapes are also possible, e.g., sinusoidal shaped grooves, and the physics is essentially the same.
Such “traditional gratings” were initially made of glass with grooves, and a few are still produced in this manner today. This, however, has a number of disadvantages. For instance, the density of the grooves is limited by the capability of the ruling engine, and the quality of the grooves produced tends to decrease as elements of the ruling engine wear from usage. Production of this type of gratings is time consuming and difficult, and the cost of such gratings is therefore high.
Molded and holographic gratings were invented later on, and their production cost is significantly lower than for glass gratings. Unfortunately, although suitable for many applications, these gratings tend to deteriorate in harsh environments. For example, in fiber optic communications, all optical components must operate for long periods of time in temperatures ranging from sub-zero to over eighty degrees Centigrade, and in humidity ranging from zero to 100 percent (see e.g., GR-468-CORE, Generic Reliability Assurance Requirements for Optoelectronic Devices Used In Telecommunications Equipment).
As can also be seen in FIG. 1a-b, traditional gratings have the property that light has to shine on the grating surface from above. This limits the useful diffraction effect of such gratings to only one dimension, and multiple units need to be assembled if multiple dimensions (axes of direction) are required.
One example of an application where the need to work with multiple wavelengths and axes is common, and growing, is wavelength division multiplexing and de-multiplexing (collectively, WDM) in fiber optic communications. The use of traditional gratings in WDM usually requires either adhesives or mechanical fixtures to keep the assembly intact. Alignment is also needed to make sure that the gratings diffract light in the proper directions. The resulting assemblies formed with such traditional gratings thus tend to be significantly larger than the optical fibers being worked with and mechanical connectors are needed for connection. All of these considerations, and others, increase the cost in a fiber optic communications system.
A relatively recent invention is the fiber Bragg grating. The fiber Bragg grating is a periodic perturbation in the refractive index which runs lengthwise in the core of a fiber waveguide. Based on the grating period, a Bragg grating reflects light within a narrow spectral band and transmits all other wavelengths which are present but outside that band. This makes Bragg gratings useful for light signal redirection, and they are now being widely used in WDM.
The typical fiber Bragg grating today is a germanium-doped optical fiber that has been exposed to ultraviolet (UV) light under a phase shift mask or grating pattern. The unmasked doped sections undergo a permanent change to a slightly higher refractive index after such exposure, resulting in an interlayer or a grating having two alternating different refractive indices. This permits characteristic and useful partial reflection to then occur when a laser beam transmits through each interlayer. The reflected beam portions form a constructive interference pattern if the period of the exposed grating meets the condition:2*Λ*neff=λwhere Λ is the grating spacing, neff is the effective index of refraction between the unchanged and the changed indices, and λ is the laser light wavelength.
FIG. 2 (background art) shows the structure of a conventional fiber Bragg grating 1 according to the prior art. A grating region 2 includes an interlayer 3 having two periodically alternating different refractive indices. As a laser beam 4 passes through the interlayer 3 partial reflection occurs, in the characteristic manner described above, forming a reflected beam 5 and a passed beam 6. The reflected beam 5 thus produced will include a narrow range of wavelengths. For example, if the reflected beam 5 is that being worked with in an application, this separated narrow band of wavelengths may carry data which has been superimposed by modulation. The reflected beam 5 is stylistically shown in FIG. 2 as a plurality of parts with incidence angles purposely skewed to distinguish the reflected beam 5 from the laser beam 4. Since the reflected beam 5 is merely directed back in the direction of the original laser beam 4, additional structure is usually also needed to separate it for actual use.
Unfortunately, as already noted, conventional fiber Bragg gratings and the processes used to make them have a number of problems which it is desirable to overcome. For example, the fibers usually have to be exposed one-by-one, severely limiting mass-production. Specialized handling during manufacturing is generally necessary because the dosage of the UV exposure determines the quality of the grating produced. The orientation of the fiber is also critical, and best results are achieved when the fiber is oriented in exactly the same direction as the phase shift mask. The desired period of the Bragg grating will be deviated from if the fiber is not precisely aligned, and accomplishing this, in turn, introduces mechanical problems. Thus, merely the way that the fiber work piece is held during manufacturing may produce stresses that can cause birefringes to form in the fiber and reduce the efficiency of the end product grating.
Once in use, conventional fiber Bragg gratings may again require special handling. The thermal expansion coefficient of the base optical fiber is often significant enough that changing environmental conditions can cause the fiber to either expand or shrink to the extent that the period of the grating and its center wavelength shift.
From the preceding discussion of traditional and fiber Bragg gratings it can be appreciated that there is a need for optical gratings which are better suited to the growing range of grating applications.
Accordingly, it is an object of the present invention to provide optical gratings which are improved with respect to the basic ease of use and range of potential uses, and to robustness and reliability once in use.
Briefly, one preferred embodiment of the present invention is an optical grating with a background region of a first material, having a first refractive index, and a grid of cells within the background region. The cells are of a second material having a second refractive index. A plurality of the cells each have at least one incident surface, and also each have opposed surfaces respective to the incident surfaces. The incident surfaces are pitched such that, when the optical grating receives a light beam, first portions thereof may strike the incident surfaces, enter the cell, travel to the opposed surfaces, be reflected there from, travel back to the incident surfaces, and exit the cell as refracted beams. The grid of cells is thus optically two-dimensional by virtue of the cells having at least two of cell-to-cell and surface-to-surface optical separations such that the reflected beams will constructively interfere for a pre-determined light wavelength when it is present in the light beam. Optionally, the grid of cells may be extended from two dimensions into a three dimensional grid or lattice of the cells.
An advantage of the present invention is that it provides improved processes to manufacture a variety of optical gratings, particularly including optical gratings able to employ Brat the Bragg effect in multiple dimensions and for up to one light wavelength per such dimension. Thus, for instance a three-dimension or cubical grating according to the present invention may separate light of one, two, or three different wavelengths (or wavelength ranges if “chirping” is also used).
Another advantage of the invention is that it separates such light wavelengths in a more useful way then prior art systems, particularly being able to direct separated narrow-band light wavelengths perpendicular to the paths of the entering beam and any exiting beam of non-separated wavelengths. Furthermore, when multiple light wavelengths are separated, their beams are inherently also widely separated apart.
Another advantage of the invention is that it may employ already well known and widely used manufacturing processes and materials, adopted from conventional electronic semiconductor integrated circuit (IC) and micro electro-mechanical system (MEMS) manufacturing. Highly desirable attributes of such processes may thus be imparted to the inventive processes and the products produced there with, including mass automated manufacturing, rigorous quality control, high yields, and low cost.
Another advantage of the invention, coincident with the above advantage, is that the invention is integratable directly into products by conventional IC and MEMS manufacturing processes. Notable also in this regard, the invention may be constructed of similar size to the devices it may typically be integrated with, and thus avoid the problems attendant with providing and using connectors, adapters, etc.
Another advantage of the invention is that it does not use fiber based media, thus eliminating a number of disadvantages in both fiber based manufacturing processes and fiber media based grating products.
And another advantage of the invention, coincident with the above advantages, is that the invention is economically in comparison to prior art optical gratings.
These and other objects and advantages of the present invention will become clear to those skilled in the art in view of the description of the best presently known mode of carrying out the invention and the industrial applicability of the preferred embodiment as described herein and as illustrated in the several figures of the drawings.